High average power fluid media lasers operating at a high pulse repetition rate generally require that the active fluid laser medium be replaced between pulses, so that energy per pulse and beam quality do not degrade as a result of thermal energy deposited in the active medium by the pumping source. As the pulse repetition rate is increased, the volume of fluid which must be replaced per unit time is proportionally increased, so that for a given required output pulse energy from a fluid medium, a transverse flow is of great advantage. It has been found that fluid lasers such as dye lasers, may have the fluid confined in a rectangular cross section waveguide flow channel. The active region is thus in the configuration of a thin slab with rectangular cross section. The waveguide flow channel is of advantage in avoiding thermal gradients in the medium characteristic of other transverse flow lasers. This minimizes the effects of refractive index gradients in the medium across the thin dimension of the slab. See, for example, Journal of Applied Physics, Volume 11, pages 1 through 33 (1976), John J. Degnan and entitled "The Wave Guide Laser: A Review".
Although waveguide lasers have been used to advantage to avoid these thermal gradient problems, excessive vertical divergence occurs in the output beam because of the existence of discrete bands of radiation running horizonally across the exit faces of the waveguide, caused by internal, standing wave patterns. The presence of many of these thin horizontal bands results in a divergence far greater than the divergence characteristic of the whole slab vertical dimension. This in turn prevents waveguide lasers from every approaching diffraction-limited performance in the vertical dimension.
Obtaining diffraction-limited operation is important in laser isotope separation processes. One such process is described in U.S. Pat. Nos. 3,772,519; 3,939,354; 4,111,531; and 4,181,898. In the technique described in these patents, an environment containing a plurality of uranium isotopes is irradiated with laser radiation of a particular frequency to selectively excite the particles of the desired isotope type. When certain particles are selectively excited, the selectively excited particles may be separated as disclosed in these patents. For optimum radiation absorption efficiency, the laser isotope separation process utilizes long channels of vapor. This requires that all the radiation from the laser be completely coupled into the region at which the isotope separation is being accomplished and that none of the radiation be lost through divergence. It is particularly detrimental for the radiation to strike the channel walls of the isotope separation reaction zone. Beam collimation is thus critical to the process.